Pressure-sensitive adhesive curable with active energy ray and pressure-sensitive adhesive sheet

ABSTRACT

The present invention relates to a pressure-sensitive adhesive curable with active energy beams, which is useful as a pressure-sensitive adhesive and pressure-sensitive adhesive sheet. It is liquid at ordinary temperature, undergoes practical crosslinking and curing in the absence of a photopolymerization initiator even when irradiated with visible light or ultraviolet light, and gives a cured coating film which suffers no discoloration and is excellent not only in various pressure-sensitive adhesive performances but in water resistance and heat resistance. The adhesive contains a compound which has two or more maleimide groups represented by the following formula (1) and is liquid at ordinary temperature:  
                 
 
where in formula (1), R 1  represents alkyl, aryl, arylalkyl, or halogen.

TECHNICAL FIELD

The present invention relates to a pressure-sensitive adhesive curablewith active energy beams, which is composed of a compound containing twoor more maleimide groups having a specific structure, and apressure-sensitive adhesive sheet having an adhesive layer formed of theabove described pressure-sensitive adhesive, which can be utilized withadmiration in the relevant technical fields.

BACKGROUND ART

Pressure-sensitive adhesives are processed into pressure-sensitiveadhesive products and used for various applications such aspressure-sensitive adhesive tapes, pressure-sensitive adhesive labelsand pressure-sensitive adhesive double coated tapes; in these years,their further improvement in performance is demanded. Particularly, foruse in adhesion of interior or exterior parts of vehicles, electricappliances and houses, mutual adhesion of electronic parts or the like,they are required to have excellent water resistance and highpressure-sensitive adhesiveness, namely, performances such that they canadhere under low compression at room temperature but do not peel offwhen allowed to stand at high temperature.

Additionally, pressure-sensitive adhesives used for first aid plasters,magnetic plasters, surgical tapes, wound-healing dressing materials(dressings), athletic tapes, tape preparations for transdermaltherapeutic systems and the like are required to be high in safetybecause these articles are directly affixed to the skin.

For use in which heat resistance is required, pressure-sensitiveadhesive products using silicone based pressure-sensitive adhesives havebeen investigated, but have not yet been widely used because they areproblematic in that there are no excellent release materials to beassociated with these products, and require use of expensive rawmaterials, thereby increasing the cost.

On the other hand, pressure-sensitive adhesives that are widely used aresolvent based pressure-sensitive adhesives comprising acrylic polymersand rubber-based resins including SIS and SEBS dissolved in organicsolvents; these pressure-sensitive adhesives are excellent in waterresistance, and additionally excellent in balance of adhesiveproperties, and thus various investigations have been performed so as tomeet the above described properties.

However, there is a fear that the solvent based pressure-sensitiveadhesives might have an adverse environmental effect due to organicsolvents evolved when processed into pressure-sensitive adhesiveproducts, and additionally have an adverse effect on safety due toresidual solvents contained in coating films of the pressure-sensitiveadhesives. Thus, in these years, pressure-sensitive adhesives using noorganic solvents come to be demanded.

As an alternative for solvent based pressure-sensitive adhesives, activeenergy beam-curable pressure-sensitive adhesives, which are curable withactive energy beams such as visible and ultraviolet light, have beeninvestigated because they are excellent in water resistance and heatresistance, easy to handle owing to low viscosity, and adaptable tocoating devices that have been used for the solvent based adhesives.

Generally, compositions curable with active energy beams includingpressure-sensitive adhesives curable with active energy beams are notcrosslinked or curable as raw materials per se, and hence it isnecessary to add a photopolymerization initiator or a photosensitizer(hereinafter, these are collectively referred to as photopolymerizationinitiator or the like).

The greater the addition amount of the photopolymerization initiator orthe like is, the quicker the curing is; and thus, the addition amount isapt to be increased. However, the photopolymerization initiator or thelike includes a compound having an aromatic ring so as to absorb lightefficiently, thereby causing a problem of yellowing of the resultantcured films.

Additionally, the photopolymerization initiator or the like is usuallycomposed of a low molecular weight compound so as to efficientlyinitiate polymerization reaction. However, when such a composition isirradiated with an active energy beam, the temperature is elevated owingto polymerization heat, and thus the low molecular weightphotopolymerization initiator or the like, which is high in vaporpressure, causes problems such that malodor is generated markedly at thetime of curing, thereby causing a problem of working environment, or theobtained products are contaminated. Additionally, there are problems inthat decomposition products including an unreacted photopolymerizationinitiator and the like remain in the cured films, and hence, when thecured films are subjected to light or heat, there often occurs a problemsuch that the cured films turn yellow, or unfavorable odor is generated.

Furthermore, when the cured films are allowed to stay in water or thelike, or the cured films are touched with sweat and the like excretedfrom human body, the unreacted photopolymerization initiator or the likebleeds out in a large quantity, and hence there occurs a problem ofsafety and health.

For the purpose of overcoming the drawbacks of the active energybeam-curable compositions that contain the photopolymerization initiatoror the like, there have been investigated compositions containingneither photopolymerization initiator nor the like but curable byirradiation with active energy beams (for example, Japanese PatentLaid-Open Nos. H11-124403A, H11-124404A, 2001-219508A and 2001-220567A).

However, when compositions that contain compounds having maleimidegroups disclosed in Japanese Patent Laid-Open Nos. H11-124403A andH11-124404A are used as pressure-sensitive adhesives, they often causegelation if they are stored at a high temperature, or the cured filmsare poor in water resistance, namely, they deteriorate in adhesivestrength and peel off if they are soaked in warm water for a long time.

On the other hand, compositions that contain compounds having maleimidegroups disclosed in Japanese Patent Laid-Open Nos. 2001-219508A and2001-220567A are both adhesives the cured coating films of which have notackiness or are poor in tackiness, so that they are hardly usable aspressure-sensitive adhesives. Additionally, the maleimidegroup-containing compound disclosed in Japanese Patent Laid-Open No.2001-220567A is crystalline with a melting point of 40° C. or above, andis not liquid at ordinary temperature, so that it is particularly poorin tackiness and hardly usable as a pressure-sensitive adhesive.

The present inventors have made intensive researches for the purpose offinding an active energy beam-curable pressure-sensitive adhesive whichis liquid at ordinary temperature, has a practical crosslinking propertyor curability in the absence of photopolymerization initiators even whenirradiated with visible or ultraviolet light, and provides cured filmsthat do not change color but are excellent in water resistance and heatresistance as well as various pressure-sensitive adhesive performances.

DISCLOSURE OF THE INVENTION

The present inventors have performed various investigations for thepurpose of solving the above described problems, and consequently haveaccomplished the present invention by finding out that apressure-sensitive adhesive containing a compound having two or moremaleimide groups can solve the above described problems.

Hereinafter, detailed description of the present invention will be made.

1. A Compound Having Two or More Maleimide Groups Represented by Formula(1)

The pressure-sensitive adhesive of the present invention requires, as anindispensable component, a compound (hereinafter simply referred to as amaleimide compound) having two or more maleimide groups, represented bythe following formula (1):

Here, in formula (1), R¹ represents an alkyl group, an aryl group, anarylalkyl group or a halogen atom.

In formula (1), as the alkyl group of R¹, an alkyl group having 4 orless carbon atoms is preferable; the aryl includes phenyl group and thelike; the arylalkyl group includes benzyl group and the like; and thehalogen atom includes a fluorine atom, a chlorine atom, a bromine atomand an iodine atom.

As R₁, an alkyl group is preferable among the above because of excellentreactivity of the resultant pressure-sensitive adhesive and easyavailability to raw materials thereof; a methyl group is more preferablebecause it is most excellent in reactivity.

The maleimide compounds used in the present invention contain the abovedescribed maleimide groups, and accordingly, irradiation with an activeenergy beam leads to dimerization of the maleimide groups, so that themaleimide compound molecules are crosslinked to each other.Additionally, also in the case where crosslinking or curing (hereinafterthese will be collectively referred to as curing) is made with the aidof ultraviolet light or visible light, the dimerization reaction can bemade to occur with the aid of irradiation with ultraviolet light orvisible light without blending of a photopolymerization initiator or thelike, or with blending of a small amount of a photopolymerizationinitiator or the like.

Additionally, the maleimide compounds in the present invention areliquid at ordinary temperature; accordingly, handling including coatingoperation comes to be easy. On the contrary, compounds which are solidat ordinary temperature are not easy to handle, and additionally, leadto a high elastic modulus of the cured film and thus to insufficientadhesive performance. Meanwhile, it should be noted that ordinarytemperature referred to in the present invention means 25° C.

The molecular weight of the maleimide compound is preferably 2,000 to20,000 in terms of number average molecular weight, more preferably3,000 to 10,000, and further preferably 3,000 to 8,000. When the numberaverage molecular weight of the maleimide compound is less than 2,000,the pressure-sensitive adhesive strength and tackiness of the cured filmare lowered as the case may be, while when the number average molecularweight exceeds 20,000, the viscosity of the pressure-sensitive adhesivebecomes too high, and coatability is lowered as the case may be.

Here, it should be noted that the number average molecular weightreferred to in the present invention is a value converted from themolecular weight measured by gel permeation chromatography (hereinafterabbreviated as GPC) by use of tetrahydrofuran as solvent with referenceto the molecular weight of polystyrene.

As the maleimide compounds to be used in the present invention, variouscompounds can be used as far as the compounds have the above describedmaleimide groups. As the maleimide compounds, compounds prepared bymeans of various methods can be used; however, the following 3 types ofcompounds are preferable because of easiness in preparation.

(1) An addition reaction product (hereinafter referred to as compound(1)) between a prepolymer having two or more isocyanate groups atterminals and a compound having a maleimide group and an active hydrogengroup.

(2) An esterification reaction product (hereinafter referred to ascompound (2)) between a prepolymer having two or more carboxylic groupsat terminals and a compound having a maleimide group and an activehydrogen group.

(3) An esterification reaction product (hereinafter referred to ascompound (3)) between a prepolymer having two or more hydroxy groups atterminals and a carboxylic acid having a maleimide group.

Now, description will be made below of the compounds (1) to (3).

1-1. Compound (1)

The compound (1) is an addition reaction product between a prepolymer(hereinafter simply referred to as urethane prepolymer) having two ormore isocyanate groups at terminals and a compound (hereinafter simplyreferred to as a maleimide/active hydrogen compound) having a maleimidegroup and an active hydrogen group, and is prepared by reacting two ormore moles of a maleimide/active hydrogen compound with 1 mole of aurethane prepolymer.

Description will be made below of the urethane prepolymer and themaleimide/active hydrogen compound.

A) Urethane Prepolymer

As the urethane prepolymer, various compounds can be used as far as thecompounds each have two or more isocyanate groups at terminals of themolecule thereof.

The urethane prepolymer includes a reaction product between a polyolhaving two or more hydroxy groups (hereinafter simply referred to aspolyol) and a polyisocyanate having two or more isocyanate groups(hereinafter simply referred to as polyisocyanate), and the like.

a1) Polyol

The polyol includes polyester polyol, polyether polyol, polymer polyolprepared from radically polymerizable monomers, and the like. Amongthese, polyester polyol is preferable because the resultant cured filmis low in viscosity, and the resultant pressure-sensitive adhesive isexcellent in water resistance.

As the polyol, two or more types of polyols can be used as required.

a1-1) Polyester Polyol

A polyester polyol is a random condensation copolymer between apolycarboxylic acid and a polyhydric alcohol. Among the polyesterpolyols, an aliphatic polyester polyol is preferable because ofexcellent curability by active energy beams of the resultantpressure-sensitive adhesive.

In this connection, as the polycarboxylic acid, various polycarboxylicacids can be used as far as the polycarboxylic acids each have two ormore carboxyl groups in molecule thereof. Specific examples includesuccinic acid, adipic acid, pimelic acid, suberic acid, azelaic acid,sebacic acid, dodecanedioic acid, icosanedioic acid, 2,6-naphthalenedicarboxylic acid, trimellitic acid, glutaric acid, 1,9-nonanedicarboxylic acid, 1,10-decane dicarboxylic acid, malonic acid, fumaricacid, 2,2-dimethylglutaric acid, 1,3-cyclopentane dicarboxylic acid,1,4-cyclohexane dicarboxylic acid, 1,3-cyclohexane dicarboxylic acid,itaconic acid, maleic acid, 2,5-norbornane dicarboxylic acid,1,4-terephthalic acid, 1,3-terephthalic acid, dimeric acid andparaoxybenzoic acid.

Among these, aliphatic dicarboxylic acids are preferable, and adipicacid and sebacic acid are more preferable.

Two or more types of the polycarboxylic acids can be used incombination, as required.

As the polyhydric alcohol, various compounds can be used as far as thecompounds each have two or more hydroxy groups in molecule thereof.Specific examples thereof include butylethylpropanediol,2,4-diethyl-1,5-pentanediol, 3-methyl-1,5-pentanediol andpolyethyleneglycol, ethyleneglycol, diethyleneglycol, triethyleneglycol,tetraethyleneglycol, 1,3-propanediol, 2-butyl-2-ethyl-1,3-propanediol,1,2-butanediol, 1,3-butanediol, 1,4-butanediol, 1,5-pentanediol,1,6-hexanediol, 1,7-heptanediol, 1,2-octanediol, 1,8-octanediol,1,9-nonanediol, 1,2-decanediol, 1,10-decanediol,2,2-dimethyl-1,3-propanediol, 2,2,4-trimethyl-1,6-hexanediol,1,3-cyclohexanedimethanol and 1,4-cyclohexanedimethanol, dimeric aciddiol and 2-methyl-1,8-octanediol.

Among these, aliphatic diols are preferable, and furthermorebutylethylpropanediol, 2,4-diethyl-1,5-pentanediol,3-methyl-1,5-pentanediol, dimeric acid diol and 2-methyl-1,8-octanediolare more preferable because the resultant pressure-sensitive adhesivesare low in viscosity and excellent in pressure-sensitive adhesivenessand water resistance.

Two or more types of polyhydric alcohols can be used in combination, asrequired.

As the maleimide compound of the present invention, as will be describedlater, a compound having a polyester skeleton is preferable, anddetailed description will be made below of the method for preparing thepolyester polyol that is the raw material for preparing the maleimidecompound.

The method for preparing the polyester polyol can be in accordance withgeneral esterification methods, and examples of the method include amethod in which in the presence of a catalyst, a polycarboxylic acid anda polyhydric alcohol are heated under stirring, and the like.

As the catalyst, catalysts usually used in the esterification reactioncan be used, and examples thereof include base catalysts, acid catalystsand metal alkoxides. Examples of the base catalyst include metalhydroxides such as sodium hydroxide and potassium hydroxide, and aminessuch as triethylamine, N,N-dimethylbenzylamine and triphenylamine.Examples of the acid catalyst include sulfuric acid andparatoluenesufonic acid. As the metal alkoxide, alkoxides of titanium,tin or zirconium are preferable. Specific examples of these metalalkoxides include tetraalkyl titanates such as tetrabutyl titanate; tinalkoxides such as dibutyltin oxide and monobutyltin oxide; and zirconiumalkoxides such as zirconium tetrabutoxide and zirconium isopropoxide.

When an obtained pressure-sensitive adhesive is applied to bonding ofpacking materials for food and related matter, it is preferable that theaddition amount of these catalysts is as small as possible.

Furthermore, among the above described catalysts, the titanium compoundsare almost free from toxicity exhibited by heavy metals and can producehigh molecular weight polyesters of practical use, and thus can bepreferably used for the bonding of packing materials for food andrelated matter.

The reaction temperature and time in the esterification reaction may beappropriately set according to purposes. The reaction temperature ispreferably 80 to 220° C.

As the aliphatic polyester polyols, commercially available ones can beused; examples thereof include “KURAPOL P-5010” manufactured by KurarayCo., Ltd., “KYOWAPOL 5000 PA” and “KYOWAPOL 3000 PA” manufactured byKyowa Hakko Kogyo Co., Ltd., and “DYNACOLL 7250” manufactured by DegussaJapan Co., Ltd.

a1-2) Polyether Polyol

The polyether polyol include polyalkyleneglycols such aspolyethyleneglycol, polypropyleneglycol, polybutyleneglycol andpolytetramethyleneglycol; ethylene oxide-modified products, propyleneoxide-modified products, butylene oxide-modified products andtetrahydrofuran-modified products of alkyleneglycols such asethyleneglycol, propanediol, propyleneglycol, tetramethyleneglycol,pentamethyleneglycol, hexanediol, neopentylglycol, glycerin,trimethylolpropane, pentaerythritol, diglycerin, ditrimethylolpropaneand dipentaerythritol; copolymers of ethylene oxide and propylene oxide,copolymers of propyleneglycol and tetrahydrofuran and copolymers ofethyleneglycol and tetrahydrofuran; hydrocarbon based polyols such aspolyisopreneglycol, hydrogenated polyisopreneglycol, polybutadieneglycoland hydrogenated polybutadieneglycol; andpolytetramethylenehexaglycerylether (tetrahydrofuran-modified productsof hexaglycerin).

a1-3) Polymer Polyol

As the polymer polyol prepared from radically polymerizable monomers,mention may be made of polymers composed of monomers having anethylenically unsaturated group and a hydroxy group as an indispensablecomponent. Specific examples include those obtained by polymerizingradically polymerizable monomers, for example, hydroxyalkyl(meth)acrylates such as hydroxyethyl (meth)acrylate and hydroxypropyl(meth)acrylate with another (meth)acrylates.

The method for preparing the polymer polyol, includes methods in whichradically polymerizable monomers are subjected to solutionpolymerization or high temperature continuous polymerization.

a2) Polyisocyanate

As the polyisocyanate, various compounds can be used as far as thecompounds each have two or more isocyanate groups in molecule thereof.Specific examples include p-phenylene diisocyanate, 4,4′-diphenylmethanediisocyanate, tolylene diisocyanate, 4,4′-diphenylene diisocyanate,1,5-octylene diisocyanate, trimethylene diisocyanate, tetramethylenediisocyanate, hexamethylene diisocyanate, pentamethylene diisocyanate,1,3-cyclopentane diisocyanate, 1,4-cyclohexane diisocyanate,4,4′-methylene bis(cyclohexyl isocyanate), methyl-2,4-cyclohexanediisocyanate, methyl-2,6-cyclohexane diisocyanate, diphenylmethanediisocyanate, 1,4-bis(isocyanatomethyl)cyclohexane,1g3-bis(isocyanatomethyl)cyclohexane, isophorone diisocyanate andcarbodiimide-modified 4,4′-diphenylmethane diisocyanate.

Among these, alicyclic and aliphatic isocyanates are preferable becausethey are excellent in curability of the resultant pressure-sensitiveadhesives with active energy beams and in weather resistance of thecured products. As the alicyclic or aliphatic isocyanates, hexamethylenediisocyanate and isophorone diisocyanate are preferable.

Two or more types of polyisocyanates can be used in combination, asrequired.

In the present invention, when the urethane prepolymer is prepared, theamount of the polyisocyanate in relation to the polyol falls within therange preferably from 1.8 to 3 and more preferably from 2 to 2.5 interms of the equivalence ratio of the group —NCO/the group —OH.

a3) Method for Preparing Urethane Prepolymer

The method for preparing the urethane prepolymer can follow theconventional methods. Examples of the methods include a method in whichpolyol and polyisocyanate are heated in the presence of a catalyst.

As the catalyst, the catalysts commonly used in urethanization reactioncan be used; examples thereof include metal compounds and amines.Examples of the metal compounds include tin based catalysts such asdibutyltin laurate and tin dioctylate; lead based catalysts such as leaddioctylate; zirconium based catalysts such as K-KAT XC-4025 and K-KATXC-6212 (manufactured by King Industries, Inc.); aluminum basedcatalysts such K-KAT XC-5217 (manufactured by King Industries, Inc.);and titanate based catalysts such as tetra-2-ethylhexyl titanate.Examples of the amines include triethylamine, N,N-dimethylbenzylamine,triphenylamine and triethylenediamine.

Additionally, when the urethane prepolymer is prepared, generally-usedradical polymerization inhibitors such as hydroquinone and triethylaminecan be used, as required, for the purpose of preventing gelation in thecourse of the reaction.

Furthermore, when the urethane prepolymer is prepared, phosphoruscompounds can be blended. By blending phosphorus compounds, the actionof the catalyst used in esterification and ring-opening polyaddition canbe brought to a halt. If the catalyst is not deactivated, esterificationreaction often occurs, when the obtained urethane prepolymer is storedin the presence of moisture or heated in the presence of moisture in asubsequent reaction, or when the resultant maleimide compound and curedproduct of the pressure-sensitive adhesive are stored in the presence ofmoisture, and thereby the physical properties of the pressure-sensitiveadhesive are markedly degraded.

As the phosphorus compounds, mention may be made of inorganic or organicphosphorus compounds and the like listed in the following (a) to (e).

(a) Phosphoric Acid and the Alkyl Esters Thereof

The phosphoric acid alkyl esters include trialkyl esters such astrimethyl phosphate, triethyl phosphate, tributyl phosphate, trinonylphosphate, triphenyl phosphate and the like.

(b) Organic Esters of Phosphonic Acid

It includes dibutylbutyl phosphonate and the like.

(c) Phosphorous Acid

Phosphorous acid is used alone or in combination with other phosphoruscompounds, which has the strongest effects of stabilizing hue andpreventing oxidative degradation.

(d) Organic Esters of Phosphorous Acid

They include dibutyl hydrogen phosphate, triphenyl phosphite and thelike. However, triphenyl phosphite sometimes degrades the properties ofthe polyester skeleton in the maleimide compound, and accordingly it isnecessary to pay attention to the addition amount thereof.

(e) Other Inorganic Phosphorus Compounds

It includes polyphosphoric acid and the like.

The amount of a phosphorus compound to be added may be appropriately setaccording to molecular weight of the phosphorus compound (content ofphosphorus atom); generally, the amount is preferably 0.001 to 3 partsby mass, and more preferably 0.01 to 1 part by mass, in relation to 100parts by mass of the polyester polyol. When the addition amount of aphosphorus compound is less than 0.001 part by mass, no effect resultingfrom the addition can be found, while when the addition amount is largerthan 3 parts by mass, no further increase of the effect is expected.

B) Maleimide/Active Hydrogen Compound

As the maleimide/active hydrogen compound, an alcohol having a maleimidegroup (hereinafter referred to as a maleimide alcohol) is preferable. Asthe maleimide alcohol, mention may be made of the maleimide alcoholrepresented by the following formula (2) and the like.

In formula (2), R¹ represents the same groups as described above, R²represents an alkylene group and preferably a straight chain or branchedchain alkylene group having 1 to 6 carbon atoms.

As the compound (1), a maleimide compound having a polyester skeleton,prepared by use of a urethane prepolymer the raw material of which is apolyester polyol, is preferable because it excels in curability withactive energy beams and in water resistance of the cured film.

C) Method for Preparing the Maleimide Compound

As for the method for preparing the maleimide compound, the maleimidecompound may be prepared by reacting a urethane prepolymer and amaleimide/active hydrogen compound with each other, according togenerally-employed urethanization reaction. The specific urethanizationreaction includes a method similar to that described above.

In the preparation of the maleimide compound, it is preferable that thereaction is carried out in the presence of an antioxidant for thepurpose of preventing discoloration of the resultant maleimide compound.

The antioxidant includes generally used phenol based, triphosphite basedand amine based antioxidants; for example, compounds described inJapanese Patent Publication Nos. S36-13738B, S36-20041B, S36-20042B andS36-20043B.

As the phenol based antioxidant, various types can be used, and thefollowing compounds are particularly preferable: butylhydroxytoluene,pentaerythrityl tetrakis[3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate],octadecyl-3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate and the like.

The phenol based antioxidant can be used in combination with zinc oxidefor the purpose of enhancing the effect.

The proportion of the antioxidant to be added is preferably 0.01 to 2parts by mass in relation to the 100 parts by mass of the urethaneprepolymer. When this proportion is less than 0.01 part by mass, asufficient effect of blending of the antioxidant cannot be obtained asthe case may be, while the blending with a proportion larger than 2parts by mass can be expected to provide no further increase of effect,and hence is disadvantageous in cost.

Additionally, when the maleimide compound is prepared, the abovedescribed phosphorus compound may be blended.

1-2. Compound (2)

The compound (2) is an esterification reaction product between aprepolymer having two or more carboxyl groups at terminals thereof(hereinafter simply referred to as carboxylic acid prepolymer) and amaleimide/active hydrogen compound.

The carboxylic acid prepolymer includes one prepared by use ofpolycarboxylic acids and polyhydric alcohols as described above, and ispreferably one having a polyester skeleton. The maleimide/activehydrogen compound includes the compounds similar to those describedabove.

The method for esterification between the carboxylic acid prepolymer andthe maleimide/active hydrogen compound may follow the method similar tothat described above.

The compound (2) is preferably used as a maleimide compound that isrequired to be lower in viscosity.

1-3. Compound (3)

The compound (3) is an esterification reaction product between aprepolymer having two or more hydroxy groups at terminals thereof(hereinafter simply referred to as polyol prepolymer) and a carboxylicacid having a maleimide group (hereinafter referred to as a maleimidecarboxylic acid).

The polyol prepolymer includes those as described above in connectionwith the polyester polyol, and is preferably that having a polyesterskeleton.

As the maleimide carboxylic acid, various types of compounds can beused, and the compounds represented by the following formula (3) arepreferable.

In formula (3), R¹ represents the same groups as described above, R³represents an alkylene group, and is preferably a straight chain orbranched chain alkylene group having 1 to 6 carbon atoms.

The esterification reaction between the polyol prepolymer and themaleimide carboxylic acid may follow the method as described above.

The compound (3) is preferably used as a maleimide compound that isrequired to be lower in viscosity.

1-4. Miscellaneous

Among the above described compounds (1) to (3), the compound (1) ispreferable in that the compound (1.) is faster in reaction rate, higherin yield and easier to prepare than the compounds (2) and (3). Forapplications where the maleimide compound is required to be low inviscosity, the compounds (2) and (3) are preferable.

The maleimide compound can be adjusted in terms of viscosity andfluidity as well as pressure-sensitive adhesive strength, holding powerand tackiness of the cured film, by varying copolymerization compositiondepending upon purposes.

As the maleimide compound used in the present invention, variouscompounds can be used as far as the compounds have the above describedmaleimide groups; the compounds having a polyester skeleton arepreferable because they are excellent in pressure-sensitive adhesivestrength.

As the maleimide compound, compounds other than those cited above canalso be used. For example, compounds having, in the skeleton thereof, anethylenically unsaturated group as well can also be used for the purposeof further improving the curability with irradiation of active energybeams.

2. Pressure-Sensitive Adhesive Curable with Active Energy Beams

The pressure-sensitive adhesive of the present invention requires theabove described maleimide compound as an indispensable component.

The pressure-sensitive adhesive of the present invention can be composedof various components as required, in addition to the maleimidecompound. Now, description will be made below of the respectivecomponents

2-1. Photopolymerization Initiator and the Like

As described above, the pressure-sensitive adhesive of the presentinvention is easily curable with active energy beams; even when curedwith ultraviolet light or visible light, the pressure-sensitive adhesiveof the present invention has an excellent curability without blending ofphotopolymerization initiators or with blending of a small amount of aphotopolymerization initiator, but can be blended with aphotopolymerization initiator or the like as required.

In the case where a photopolymerization initiator is blended, examplesof the photopolymerization initiator include benzoin and its alkylethers, acetophenones, anthraquinones, thioxanthones, ketals,benzophenones, xanthones, acylphosphine oxides, α-diketones and thelike.

Additionally, for the purpose of improving the sensitivity to activeenergy beams, photosenistizers can be used.

As the photosensitizer, mention may be made of benzoic acid basedphotosensitizers, amine based photosensitizers, and the like. Thesephotosensitizers can be used in combination of two or more thereof. Theblending proportion of these photosensitizers is preferably 0.01 to 10parts by mass in relation to 100 parts by mass of the maleimidecompound.

As the photopolymerization initiator, benzophenones and thioxanthonesare preferable because they are highly effective for improving thecuring rate of the compositions.

2-2. Compounds Having Reactive Unsaturated Groups

For the purpose of enhancing the pressure-sensitive adhesive performanceof the cured film, or for the purpose of adjusting the sensitivity ofthe pressure-sensitive adhesive, the present pressure-sensitive adhesivemay contain a compound having a reactive unsaturated group such as(meth)acrylic monomer, (meth)acrylic oligomer and the like.

Examples of the (meth)acrylic monomer include alkyl acrylates or alkylmethacrylates (hereinafter acrylate or methacrylate will be referred toas (meth)acrylate), hydroxyalkyl (meth)acrylates, (meth)acrylates ofphenol alkylene oxide adducts, mono or di(meth)acrylates of glycols, andpolyol poly(meth)acrylates, poly(meth)acrylates of polyol alkylene oxideadducts.

Examples of the (meth)acrylic oligomer include urethane (meth)acrylateoligomers, polyester (meth)acrylate oligomers, and epoxy(meth)acrylateoligomers.

The proportion of the compound having a reactive unsaturated group to beadded is preferably 50 parts by mass or less, and more preferably 20parts by mass or less, in relation to 100 parts by mass of the maleimidecompound.

2-3. Polymers

The pressure-sensitive adhesive of the present invention may besupplemented with a polymer for the purpose of adjusting the viscositybefore curing or adjusting the pressure-sensitive adhesive performanceafter curing. There is no particular limitation to the polymerconcerned, and examples of the polymer include (meth)acrylate basedpolymers, polystyrenes and polyolefins. Among these, (meth)acrylatebased polymers having a maleimide group represented by the belowdescribed formula (4) are preferable for the purpose of enhancing waterresistance and heat resistance of the pressure-sensitive adhesivebecause these polymers form, upon curing, crosslinkage with themaleimide compound of the present invention.

2-4. Other Maleimide Compounds

The pressure-sensitive adhesive of the present invention may contain acompound having one maleimide group as far as curability withirradiation of active energy beams and performance of the cured film arenot damaged.

Examples of the compound concerned include a compound having onemaleimide group in a skeleton as described above, a compound having amaleimide group and an ethylenically unsaturated group, and the like.

In the case where the maleimide compound of the present invention isoriginated from a polymer polyol prepared from the above describedradically polymerizable monomers, the polymer polyol is often a mixtureof a polymer polyol having two or more hydroxy groups in a molecule anda polymer polyol having one hydroxy group in a molecule. In this case,the resultant compound is a mixture of a compound having two or more ofmaleimide groups and a compound having one maleimide group, and can beused as it is.

The blending proportion of the compound having one maleimide group ispreferably 80 parts by mass or less and more preferably 50 parts by massor less, in relation to 100 parts by mass of the maleimide compound.

Additionally, as far as curability with irradiation of active energybeams and performance of the cured film are not degraded, compoundshaving maleimide groups other than the above described maleimide groupscan be used.

As compounds having such maleimide groups concerned, mention may be madeof a compound having a maleimide group represented by the belowdescribed formula (4), formula (5) or the like and having a skeleton asdescribed above, a compound having a maleimide groups represented by thebelow described formula (4), formula (5) or the like and anethylenically unsaturated group, and the like. The blending proportionsof these maleimide compounds are preferably 150 parts by mass or less,and more preferably 50 parts by mass or less, in relation to 100 partsby mass of the maleimide compound.

Here, in formula (4), R⁴ and R⁵ are each independently an alkyl grouphaving 4 or less carbon atoms, or a group fused to form a carbon ring.Examples of the group fused to form a carbon ring include the group—CH₂CH₂CH₂—, the group —CH₂CH₂CH₂CH₂—, and the like.

Among the compounds having maleimide groups other than the abovedescribed maleimide groups, compounds having two or more maleimidegroups represented by the above described formula (5) are preferablebecause these compounds can improve the curability of thepressure-sensitive adhesive. These compounds preferably have theskeleton as described above. The blending proportion of the maleimidecompound concerned is preferably 150 parts by mass or less, and morepreferably 50 parts by mass or less, in relation to 100 parts by mass ofthe maleimide compound.

2-5. Tackifiers

The pressure-sensitive adhesive of the present invention may contain atackifier for the purpose of lowering glass transition temperature(hereinafter abbreviated as Tg) or enhancing pressure-sensitive adhesiveperformance of the cured film.

As the tackifier, various substances can be used; examples thereofinclude natural resins such as rosin based resins and terpene basedresins and the derivatives thereof, and synthetic resins such aspetroleum resins. Among these, preferable are those compounds which haveno double bond or are small in double bond content, because they do notinhibit the curing with active energy beams of the pressure-sensitiveadhesive.

The blending proportion of the tackifier is preferably 20 parts by massor less, and more preferably 10 parts by mass or less in relation to 100parts by mass of the maleimide compound. When the blending proportion ofthe tackifier exceeds 20 parts, the viscosity of the pressure-sensitiveadhesive becomes too high, and accordingly, the coatability and the heatresistance are degraded as the case may be.

2-6. Crosslinkers

The pressure-sensitive adhesive of the present invention may contain acrosslinker capable of reacting rapidly at ordinary temperature, for thepurpose of forming crosslinkage between polymer molecules and enhancingheat resistance and water resistance. Examples of the crosslinkerinclude polyisocyanate compounds, polyoxazoline compounds, epoxy resins,aziridine compounds, polycarbodimaleimide compounds and coupling agents.

2-7. Acid Masking Agents

When the maleimide compound is derived from a polyester, it ispreferable that an acid masking agent is blended in thepressure-sensitive adhesive.

The ester bonds abundant in such pressure-sensitive adhesives arehydrolyzed by the moisture and the molecular weight thereof is therebylowered, in the case where the pressure-sensitive adhesive is used overa long period of time under a harsh condition such as high temperatureor humidity; additionally, the acidic carboxylic groups generated by thehydrolysis further promote the hydrolysis of the ester bonds, and as isknown, there are problems in that the pressure-sensitive adhesivestrength is lowered, and when a pressure-sensitive adhesive article ispeeled off, the lowered pressure-sensitive adhesive strength causesproblems such that the pressure-sensitive adhesive layer undergoes thecohesion failure and after peeling off, the adhesive deposit is left onthe matter that have been coated with the pressure-sensitive adhesiveconcerned.

In this case, blending of the acid masking agent can trap the carboxylicgroups generated by the hydrolysis so that the hydrolysis can beprevented from further proceeding.

As the acid masking agent, mention may be made of carbodiimidecompounds, oxazoline compounds, epoxy compounds, and the like.

The blending proportion of the acid masking agent is preferably 0.1 to 2parts by mass in relation to 100 parts by mass of the maleimidecompound. When this proportion is less than 0.1 part by mass, nosufficient effect of blending the acid masking agent is displayed as thecase may be, while even when the blending proportion exceeds 2 parts bymass, no further effect can be expected, disadvantage to the costthereby arises, and the masking agent exhibits a plasticizing effect,thereby causing the lowering of the pressure-sensitive adhesiveperformance of the cured product.

2-8. Ultraviolet Light Absorbers and Light Stabilizers

The pressure-sensitive adhesive of the present invention may contain anultraviolet light absorber and a light stabilizer in order to improvelight resistance.

Examples of the ultraviolet light absorber include benzotriazole basedultraviolet light absorbers, and more specifically,2-(5-methyl-2-hydroxyphenyl)benzotriazole and2-(3,5-di-t-amyl-2-hydroxyphenyl)benzotriazole.

Examples of the light stabilizer include hindered amine based lightstabilizers and benzoate based light stabilizers. Examples of thehindered amine based light stabilizers includebis(1,2,2,6,6-pentamethyl-4-piperidinyl)sebacate andbis(1,2,2,6,6-pentamethyl-4-piperidinyl)2-(3,5-di-t-butyl-4-hydroxybenzyl)2-n-butylmalonate. Examples of thebenzoate based light stabilizer include2,4-di-t-butylphenyl-3,5-di-t-butyl-4-hydroxybenzoate.

2-9. Miscellaneous

The pressure-sensitive adhesive of the present invention may contain afiller for the purpose of coloring and enhancing adhesive performance.Specific examples of the filler include various types of silica, dyes,calcium carbonate, magnesium carbonate, titanium oxide, iron oxides,glass fibers, and the like.

Additionally, according to use, radical polymerization inhibitorsincluding hydroquinone and hydroquinone monomethyl ether may be blended.

In addition to these, the following common additives used in adhesivesor pressure-sensitive adhesives can be simultaneously used in theproportions usually applied: antifoamers, dyes and pigments, thickeners,lubricants, film forming aids, fillers, plasticizers, antistatic agents,textile auxiliaries, detergents, antistatic agents, level dyeing agents,dispersion stabilizers, hydrophilic resins, latexes, wetting agents,leveling improvers and the like.

Additionally, for the purpose of adjusting the viscosity, organicsolvents can be blended as required.

Furthermore, the above described phosphorus compounds and antioxidantscan be blended.

3. Method for Producing Pressure-Sensitive Adhesive Sheets

The pressure-sensitive adhesive of the present invention can be used invarious uses, and is used usually in a form of a pressure-sensitiveadhesive sheet.

The pressure-sensitive adhesive sheet may be produced following theconventional methods. Examples of the preferable methods include amethod in which the pressure-sensitive adhesive of the present inventionis coated onto a substrate, and the coating concerned is irradiated withan active energy beam and thereby cured.

Examples of the substrate include metals, plastics, glass, ceramics,wood, paper, printing paper and fibers. Examples of the metals includealuminum, iron and copper; and examples of the plastics include vinylchloride polymers, acrylate based polymers, polycarbonate, polyethyleneterephthalate, acrylonitrile-butadiene-styrene copolymer, polyethyleneand polypropylene.

Examples of the coating method of pressure-sensitive adhesives includeroll coating, gravure printing, screen printing, die coating and knifecoating.

The coating quantity of a pressure-sensitive adhesive for a substratemay be appropriately chosen according to the intended use; the coatingquantity is preferably 5 to 200 g/m², and more preferably 10 to 100g/m². When the coating quantity is less than 5 g/m², thepressure-sensitive adhesive strength often comes to be insufficient,while when the coating quantity is 200 g or more, the active energy beamcan hardly reach the deep portion and accordingly the intendedperformance cannot be obtained as the case may be.

After the completion of the coating process, an active energy beam isirradiated and the maleimide groups of the maleimide compound arethereby crosslinked to each other to increase the molecular weight, sothat the cohesion force and the pressure-sensitive adhesive strength ofthe obtained cured film are enhanced.

The method for irradiating the active energy beam in this case mayfollow the method applied to the conventional pressure-sensitiveadhesive curable with active energy beams. Examples of the active energybeam include visible light, ultraviolet light, X-ray and electron beam;it is preferable to use ultraviolet light, for which an inexpensivedevice can be used. Examples of the light source for the case whereultraviolet light is used include an ultra high pressure mercury lamp, ahigh pressure mercury lamp, a medium pressure mercury lamp, a lowpressure mercury lamp, a metal halide lamp, a xenon lamp, anelectrodeless discharge lamp and a carbon arc lamp; it is sufficient toirradiate for a few seconds to a few minutes.

Additionally, the Tg of the cured film made of the pressure-sensitiveadhesive of the present invention is preferably −10° C. or below, andmore preferably −30° C. or below. Herein, it should be noted that the Tgreferred to in the present invention means the peak temperature of thetanδ obtained by measuring the temperature dependence by use of adynamic viscoelasticity measurement apparatus.

4. Applications

The pressure-sensitive adhesive of the present invention can be used forvarious applications.

Examples of the applications include pressure-sensitive adhesiveproducts made of pressure-sensitive adhesive tapes, pressure-sensitiveadhesive labels and pressure-sensitive adhesive sheets; examples ofthese pressure-sensitive adhesive products include pressure-sensitiveadhesive sheets for use in confidential post cards, pressure-sensitiveadhesive sheets for use in wallpaper, pressure-sensitive adhesive doublecoated tapes, seals, stickers and masking films.

In particular, the pressure-sensitive cured film of the presentinvention is excellent in water resistance, and accordingly useful forproduction of pressure-sensitive adhesives for use in the applicationsrequiring water resistance, more specifically, seals for canned beveragecontainers, and advertising displays and stickers for vehicles both usedin the outdoors. Additionally, the pressure-sensitive adhesive of thepresent invention is high in safety, and hence can be preferably used asthe pressure-sensitive adhesive used for the tapes and the like directlyaffixed to the skin. Specific examples include the following articles:first aid plasters; magnetic plasters; surgical tapes used for fixinggauze or a catheter to the body; wound-healing dressing materials(dressings) used in the curing in which the wound is tightly covered forpreventing the wound from being infected from the outside,simultaneously easing the pain, and keeping the wound in a wetcondition; tapes for sport taping for protecting and reinforcing injuredportions of joints and muscle for the purpose of the prevention of theinjury at the time of exercise, the first aid treatment, the preventionof recurrence (rehabilitation), and the like; and the tape preparationsused in transdermal therapeutic systems in which medicine is absorbedthrough the skin and delivered into the body.

The advantages of the present invention are summarized as follows.According to the present pressure-sensitive adhesive curable with activeenergy beams, the adhesive concerned is liquid at ordinary temperature,so that it is easy to handle, excellent in coatability; and even whenirradiated with visible light or ultraviolet light, it has a practicalcrosslinking property and a practical curability in the absence of aphotopolymerization initiator, so that it is low in unfavorable odor andtoxicity, and safe. Additionally, the obtained cured film hascharacteristics such that it suffers from no discoloration, is excellentin various pressure-sensitive adhesive performances, and excellent inwater resistance and heat resistance.

BEST MODE FOR CARRYING OUT THE INVENTION

More specific description of the present invention will be made belowwith reference to the examples and comparative examples.

Meanwhile, it should be noted that, in the following, “parts” means“parts by mass”.

EXAMPLE 1

In a flask equipped with a stirrer, a thermometer and a condenser, 250 gof polyester polyol, namely, KYOWAPOL 5000PA (polyester diol, namely, areaction product between 2,4-diethyl-1,5-pentanediol and adipic acid,manufactured by Kyowa Hakko Kogyo Co., Ltd.) was placed at roomtemperature, heated to raise the temperature up to 120° C. while beingstirred, and dehydrated for 1 hour under reduced pressure.

After dehydration, the dehydrated matter was cooled to 80° C., combinedwith 22.8 g of isophorone diisocyanate (hereinafter referred to asIPDI), mixed together for 1 hour, and then supplemented with 0.050 g ofdi-n-butyltin dilaurate (hereinafter referred to as DBTL) to allow thereaction to proceed for 2 hours. Additionally, 15.9 g of 2-hydroxyethylcitracoimide (a compound represented by the above described formula (2)in which R¹ is methyl group and R² is ethylene group; hereinafterreferred to as CM-ETA) was added thereto, and the reaction was allowedto proceed for 2 hours to produce a maleimide compound.

The viscosity of this compound at 25° C. was 600,000 mpa-S, and thenumber average molecular weight thereof was about 6,000. This compoundis referred to as M1.

EXAMPLE 2

A maleimide compound was prepared in the same manner as in Example 1,except that 250 g of KYOWAPOL 5000PA, 16.8 g of 1,6-hexamethylenediisocyanate (manufactured by Nippon Polyurethane Industry Co., Ltd.;hereinafter referred to as HDI) in place of IPDI, 0.025 g of DBTL, and15.8 g of CM-ETA were used; and 0.1 g of butylhydroxytoluene as anantioxidant was added before CM-ETA was added.

The viscosity of this compound at 25° C. was 1,000,000 mpa.S, and thenumber average molecular weight thereof was about 6,000. This compoundis referred to as M2.

EXAMPLE 3

A maleimide compound was prepared in the same manner as in Example 1,except that KYOWAPOL 5000PA was replaced with 200 g of KYOWAPOL 2000PA(polyester diol, namely, a reaction product between2,4-diethyl-1,5-pentanediol and adipic acid, manufactured by Kyowa HakkoKogyo Co., Ltd.), and 40.7 g of IPDI, 0.038 g of DBTL and 31.8 g ofCM-ETA were used.

The viscosity of this compound at 25° C. was 900,000 mpa.S, and thenumber average molecular weight thereof was about 3,000. This compoundis referred to as M3.

EXAMPLE 4

A maleimide compound was prepared in the same manner as in Example 1,except that KYOWAPOL 5000PA was replaced with 200 g of KYOWAPOL 2000BA(polyester diol as a reaction product between butylethylpropane diol andadipic acid, manufactured by Kyowa Hakko Kogyo Co., Ltd.), and 40.7 g ofIPDI, 0.038 g of DBTL and 31.8 g of CM-ETA were used.

The viscosity of this compound at 25° C. was 7,500,000 mpa.S, and thenumber average molecular weight thereof was about 3,000. This compoundis referred to as M4.

EXAMPLE 5

M1 prepared in Example 1 and M′3 prepared in Comparative Example 3described below were blended in a mass ratio of 2:1. The viscosity ofthis composition at 25° C. was 780,000 mpa.S

EXAMPLE 6

To 100 parts by mass of Ml prepared in Example 1, 0.2 part by mass of anacid masking agent, namely, ELASTOSTAB H01 manufactured by NisshinboIndustries, Inc., which is a carbodiimide compound, and 0.01 part bymass of triphenyl phosphate (hereinafter referred to as TPP) as aphosphorus compound were added. The viscosity of this composition at 25°C. was 600,000 mpa.S.

EXAMPLE 7

To 100 parts by mass of Ml prepared in Example 1, was added 3 parts bymass of a polyether based bismaleimide acetate compound represented bythe following formula (6) (LUMICURE MIA-200 manufactured by DainipponInk & Chem. Inc.; hereinafter referred to as LUM; in formula (6), Rrepresents alkylene group, and n represents a value corresponding to anumber average molecular weight of about 1,000). The viscosity of thiscomposition at 25° C. was 550,000 mpa.S.

EXAMPLE 8

In a flask which was similar to that used in Example 1 and equipped witha Dean-Stark trap, 240 g of a polyester having carboxylic acid groups atboth ends (a reaction product between 2,4-diethyl-1,5-pentanediol andadipic acid, with a number average molecular weight of 3030 and an acidvalue of 37 KOH mg/g) was placed at room temperature, heated to raisethe temperature up to 120° C. while being stirred, and dehydrated for 1hour under reduced pressure.

The temperature was raised up to 160° C. still under reduced pressure,thereafter 27.3 g of CM-ETA and 0.28 mg of a titanate basedpolymerization catalyst, namely, tetrabutoxy titanium were addedthereto, and reaction was allowed to proceed for 20 hours to yield amaleimide compound.

The viscosity of this compound at 25° C. was 15,000 mpa.S, and thenumber average molecular weight thereof was about 3,350. This compoundis referred to as M5.

EXAMPLE 9

A maleimide compound was prepared in the same manner as in Example 1,except that KYOWAPOL 5000PA was replaced with 200 g of KURAPOL P-4050(polyester diol as a reaction product between 3-methyl-1,5-pentanedioland sebacic acid, manufactured by Kuraray Co., Ltd.), and 23 g of IPDI,0.025 g of DBTL and 16.5 g of CM-ETA were used; and 0.04 g ofpentaerythrityl-tetrakis(3-(3,5-di-t-butyl-4-hydroxyphenyl)propionate)(IRGANOX 1010, manufactured by Ciba Speciality Chemicals Ltd.) as anantioxidant and 0.1 g of TPP as a phosphorus compound were addedthereto.

The viscosity of this compound at 25° C. was 650,000 mpa.S, and thenumber average molecular weight thereof was about 5,000. This compoundis referred to as M6.

EXAMPLE 10

A maleimide compound was prepared in the same manner as in Example 1,except that 200 g of a polyester diol compound Al which had a numberaverage molecular weight of about 5,000 and was prepared from thereaction of 0.6 mol of 2,4-diethyl-1,5-pentanediol, 0.1 mol ofethyleneglycol, 0.3 mol of 1,6-hexanediol and 1 mol of sebacic acid,18.3 g of IPDI, 0.025 g of DBTL, and 13 g of CM-ETA were used; and 0.04g of BHT as an antioxidant and 0.1 g of TPP as a phosphorus compoundwere added thereto.

The viscosity of this compound at 25° C. was 550,000 mpa.S, and thenumber average molecular weight thereof was about 6,000. This compoundis referred to as M7.

EXAMPLE 11

In a flask similar to that in Example 8, 200 g of KURAPOL P-4050 wasplaced, and additionally 18.6 g of carboxymethyl citracoimide (acompound represented by the above described formula (3) in which R₁ ismethyl group and R₃ is methylene group), 300 g of toluene and 3.0 g ofparatolunensulfonic acid were placed; heating at 120° C. was carried outfor 6 hours while the toluene was being refluxed, and 1.8 g of water wasremoved by distillation.

The reaction solution was cooled, and thereafter transferred into aseparating funnel; and then the organic layer was washed 3 times byrepeating the sequence of operations of putting 200 g of water in thefunnel, sufficiently shaking the mixture, allowing the solution to standstill and separate, and then discarding the lower layer of the waterphase. The obtained organic layer was heated under reduced pressure toremove the toluene by distillation, and thus a maleimide compound wasobtained.

The viscosity of this compound at 25° C. was 35,000 mpa.S, and thenumber average molecular weight thereof was about 4,820. This compoundis referred to as M8.

Comparative Example 1

A maleimide compound different from the maleimide compound of thepresent invention was obtained by carrying out the reaction in the samemanner as in Example 2 except that in place of CM-ETA, 20.5 g of2-hydroxyethyltetrahydrophthalimide (a compound represented by thefollowing formula (7); hereinafter referred to as HT-ETA) was used.

The viscosity of this compound at 25° C. was 900,000 mpa.S, and thenumber average molecular weight thereof was about 6,000. This compoundis referred to as M′1.

Comparative Example 2

In a flask similar to that in Example 1, 150 g of a polyester, HS2H-200S (a polyester as a reaction product between sebatic acid and1,6-hexanediol, manufactured by Hokoku Corp.), was placed, anddehydration was carried out for 30 minutes in the same manner as inExample 1.

After dehydration, the reaction solution was cooled down to 80° C., and25.1 g of HDI as an isocyanate was added thereto and mixed for 30minutes. Then, 0.01 g of DBTL was added to allow the reaction to proceedfor 2 hours. Additionally, 24.3 g of CM-ETA was added to allow thereaction to proceed for 2 hours, and thus a maleimide compound wasobtained.

This compound was solid at 25° C. This compound is referred to as M′2.

Comparative Example 3

A maleimide compound different from the maleimide compound of thepresent invention was obtained by carrying out the reaction in the samemanner as that in Example 1 except that in place of CM-ETA, 24.3 g of2-hydroxyethyl maleimide (a compound represented by the followingformula (8); hereinafter referred to as MT-ETA) was used.

The viscosity of this compound at 25° C. was 800,000 mpa.S, and thenumber average molecular weight thereof was about 6,000. This compoundis referred to as M′3.

Evaluation

The compounds and compositions obtained in the above described examplesand comparative examples were used as pressure-sensitive adhesives.

Onto the surface of a 50 μm thick polyester film as a substrate sheet, apressure-sensitive adhesive was applied so as to have a thickness of 25μm, and ultraviolet light was irradiated from the coating surface sideby passing the sheet 4 times under a 120 W/cm light condensing highpressure mercury lamp (one lamp; 10 cm high) at a conveyer speed of 6.6m/min. The sheet thus obtained was cut into a 200 mm long and 25 mm widespecimen.

By use of the obtained specimen, the following tests were carried out.The results are shown in Table 1.

(1) Pressure-Sensitive Adhesive Strength

The 180 degree peeling strength of the specimen was measured in such away that under the conditions of 23° C. and 65% RH, the 100 mm longportion of the specimen was bonded to a SUS plate and was allowed tostand for 2 days, and then the strength was measured in accordance withJIS Z-0237.

(2) SAFT (Shear Adhesion Failure Temperature)

The 25 mm long specimen was bonded to a SUS plate similarly to (1), andallowed to stand for 1 day; and then a 500 g weight was suspended froman end of the specimen, the temperature was varied from 40° C. to 200°C. at a temperature rise rate of 0.4° C./min, and the weight falltemperature was derived from the time at which the weight fell. When thespecimen was retained at 200° C., the displacement distance wasmeasured.

(3) Tack

In conformity with JIS Z-0237 for the ball rolling method, measurementswere made in the atmosphere of. 23° C. and 65% RH.

(4) Water Resistance

The specimen was bonded under the conditions similar to those in theabove described (1) and allowed to stand for 1 day, and then furtherimmersed for 1 day in distilled water at 80° C.; then the specimen wastaken out and put again under the conditions of 23° C. and 65% RH, andsubsequently the pressure-sensitive adhesive strength was measured inthe same manner as in (1). The lowering ratio of the thus-obtainedpressure-sensitive adhesive strength with reference to thepressure-sensitive adhesive strength of (1) was calculated and evaluatedon the basis of the following 3 grades.

-   -   G: The lowering ratio of the pressure-sensitive adhesive        strength was less than 20%.    -   M: The lowering ratio of the pressure-sensitive adhesive        strength was 20% or more and less than 50%.    -   P: The lowering ratio of the pressure-sensitive adhesive        strength was 50% or more.        (5) Yellowing of Sheets

By visual inspection, degree of yellowing of the specimen was observed,and was evaluated on the basis of the following 3 grades.

-   -   G: No yellowing.    -   M: Slight yellowing.

P: Yellowing close to dark brown TABLE 1 Pressure-sensitive adhesiveEvaluation results Other Pressure- Maleimide maleimide State ofsensitive compound compound pressure- adhesive Water [state] [state]sensitive strength resist- Yellow- (parts) (parts) adhesive (g/inch)SAFT Tack ance ing Ex. M1 — Liquid 1,400 No 8 G G 1 [liquid]displacement Ex. M2 — Liquid 1,350 No 8 G G 2 [liquid] displacement Ex.M3 — Liquid 1,450 No 7 G G 3 [liquid] displacement Ex. M4 — Liquid 1,000No 8 G G 4 [liquid] displacement Ex. M1 M′3 Liquid 900 No 7 M G 5[liquid] [liquid] displacement  (2) (1) Ex. M1 — Liquid 1,500 No 9 G G 6[liquid] displacement Ex. M1 LUM Liquid 1,100 No 7 M G 7 [liquid] (3)displacement (100) Ex. M5 — Liquid 900 No 7 G G 8 [liquid] displacementEx. M6 — Liquid 1,300 No 8 G G 9 [liquid] displacement Ex. M7 — Liquid1,500 No 8 M G 10  [liquid] displacement Ex. M8 — Liquid 500 No 8 G G11  [liquid] displacement Com. — M′1 Liquid 2,000 70° C. 9 G G Ex.[liquid] 1 Com. — M′2 solid 0 — 2 or — G Ex. [solid] less 2 Com. — M′3Liquid 700 No 6 P G Ex. [liquid] displacement 3Note:“Ex.” means Example, and “Com. Ex.” means Comparative Examples.Industrial Applicability

The present pressure-sensitive adhesive curable with active energy beamsis easy to handle, can be crosslinked or cured in the absence of aphotopolymerization initiator or by addition of a small amount of aphotopolymerization initiator even when irradiated with visible light orultraviolet light; additionally, the obtained cured film is not colored,but excellent in various pressure-sensitive adhesive performances, andexcellent in water resistance and heat resistance. Thus, the presentpressure-sensitive adhesive is extremely suitable for applicationsutilizing pressure-sensitive adhesives and pressure-sensitive adhesivesheets.

1. A pressure-sensitive adhesive curable with an active energy beam,comprising a compound which has two or more maleimide groups representedby the following formula (1) and is liquid at ordinary temperature:

where in formula (1), R¹ represents an alkyl group, an aryl group, anarylalkyl group or a halogen atom.
 2. A pressure-sensitive adhesivecurable with an active energy beam, according to claim 1, in which saidcompound is a compound having a polyester skeleton.
 3. Apressure-sensitive adhesive curable with an active energy beam,according to claim 2, in which said compound is one or more selectedfrom the compounds described in the following (1) to (3): (1) anaddition reaction product between a polyester based prepolymer havingtwo or more isocyanate groups at terminals thereof and a compound havinga maleimide group and an active hydrogen group; (2) an esterificationreaction product between a polyester based prepolymer having two or morecarboxyl groups at terminals thereof and a compound having a maleimidegroup and an active hydrogen group; and (3) an esterification reactionproduct between a polyester based prepolymer having two or more hydroxygroups at terminals thereof and a carboxylic acid having a maleimidegroup.
 4. A pressure-sensitive adhesive curable with an active energybeam, according to any one of claims 1 to 3, in which said compound isone having a number average molecular weight of 2,000 to 20,000.
 5. Apressure-sensitive adhesive sheet, comprising a substrate and apressure-sensitive adhesive layer formed on the substrate by coatingonto the substrate a pressure-sensitive adhesive curable with an activeenergy beam, and irradiating said coating with an active energy beam tocrosslink or cure said coating, in which said pressure-sensitiveadhesive comprises a compound which has two or more maleimide groupsrepresented by the following formula (1) and is liquid at ordinarytemperature:

where in formula (1), R¹ represents an alkyl group, an aryl group, anarylalkyl group or a halogen atom.
 6. A pressure-sensitive adhesivesheet, according to claim 5, in which said compound is a compound havinga polyester skeleton.
 7. A pressure-sensitive adhesive sheet, accordingto claim 6, in which said compound is one or more selected from thecompounds described in the following (1) to (3): (1) an additionreaction product between a polyester based prepolymer having two or moreisocyanate groups at terminals thereof and a compound having a maleimidegroup and an active hydrogen group; (2) an esterification reactionproduct between a polyester based prepolymer having two or more carboxylgroups at terminals thereof and a compound having a maleimide group andan active hydrogen group; and (3) an esterification reaction productbetween a polyester based prepolymer having two or more hydroxy groupsat terminals thereof and a carboxylic acid having a maleimide group. 8.A pressure-sensitive adhesive sheet, according to any one of claims 5 to7, in which said compound is one having a number average molecularweight of 2,000 to 20,000.